A large proportion (some estimates are as high as twenty-five percent) of the electricity generated in the United States each year goes to lighting. Accordingly, there is an ongoing need to provide lighting which is more energy-efficient.
Efforts have been ongoing to develop ways by which solid state light emitters can be used in place of incandescent lights, fluorescent lights and other light-generating devices in a wide variety of applications. In addition, where light emitting diodes (or other solid state light emitters) are already being used, efforts are ongoing to provide light emitting diodes (or other solid state light emitters) which are improved, e.g., with respect to energy efficiency, color rendering index (CRI Ra), contrast, efficacy (lm/W), and/or duration of service.
A variety of solid state light emitters are well-known. For example, one type of solid state light emitter is a light emitting diode.
Light emitting diodes are semiconductor devices that convert electrical current into light. A wide variety of light emitting diodes are used in increasingly diverse fields for an ever-expanding range of purposes.
More specifically, light emitting diodes are semiconducting devices that emit light (ultraviolet, visible, or infrared) when a potential difference is applied across a p-n junction structure. There are a number of well-known ways to make light emitting diodes and many associated structures, and the present invention can employ any such devices. By way of example, Chapters 12-14 of Sze, Physics of Semiconductor Devices, (2d Ed. 1981) and Chapter 7 of Sze, Modern Semiconductor Device Physics (1998) describe a variety of photonic devices, including light emitting diodes.
The expression “light emitting diode” is used herein to refer to the basic semiconductor diode structure (i.e., the chip). The commonly recognized and commercially available “LED” that is sold (for example) in electronics stores typically represents a “packaged” device made up of a number of parts. These packaged devices typically include a semiconductor based light emitting diode such as (but not limited to) those described in U.S. Pat. Nos. 4,918,487; 5,631,190; and 5,912,477; various wire connections, and a package that encapsulates the light emitting diode.
As is well-known, a light emitting diode produces light by exciting electrons across the band gap between a conduction band and a valence band of a semiconductor active (light-emitting) layer. The electron transition generates light at a wavelength that depends on the band gap. Thus, the color of the light (wavelength) emitted by a light emitting diode depends on the semiconductor materials of the active layers of the light emitting diode.
Although the development of light emitting diodes has in many ways revolutionized the lighting industry, some of the characteristics of light emitting diodes have presented challenges, some of which have not yet been fully met. For example, a wide variety of lighting devices, in particular LEDs, do not operate as reliably at elevated temperatures as they do at lower temperatures. In the case of LEDs, heatsinking is often provided so that the heat generated by the LED junction is dissipated into the ambient air to keep the junction temperature of the LED low—typically, a junction temperature of 75 degrees C. is a desirable maximum. Such a requirement, however, puts a severe constraint on the amount of power that can be provided to the LEDs while dissipating enough heat to satisfy the temperature requirement, which often means that LED-containing light bulbs are dull when compared to incandescent and fluorescent counterparts.